Silver-based contact material, use of such a contact material, in switchgear for power engineering applications and method of manufacturing the contact material

ABSTRACT

Contact material based on silver, use of such a contact material in a switching device in power engineering, and process for preparing the contact material. 
     For contact pieces in low-voltage switches, in particular, substitute materials based on silver-iron oxide are proposed for the silver-nickel hitherto often used in practice. According to the invention, such a material contains, as a further effective component, an oxide of a metal of the third sub-group, yttrium oxide (Y 2  O 3 ) being especially designed for this purpose. For example, a material of the composition Ag/Fe 2  O 3  10/Y 2  O 3  1 meets, with its favourable temperature behaviour, the properties required with respect to the contact property spectrum. In addition, at least one metal oxide which contains elements of the sixth sub-group of the Periodic Table of the Elements, preferably iron tungstate (FeWO 4 ), can be present. In particular, a material of the composition Ag/Fe 2  O 3  9/Y 2  O 3  1/FeWO 4  0.5 has shown good utility owing to its additionally improved welding and short-circuit behaviour.

BACKGROUND OF THE INVENTION

The invention relates to a contact material based on silver, there beingpresent, in addition to silver, iron oxide as the main effectivecomponent and at least one further effective component. The inventionalso relates to the use of such a contact material in a switching devicein power engineering and to the associated process for preparing thecontact material.

Known contact materials for contact pieces in low-voltage switchingdevices in power engineering, e.g. in circuit breakers and in D.C.contactors, motor contactors and contactor relays include those, on theone hand, of the system silver-metal (AgMe) and, on the other hand, ofthe system silver-metal oxide (AgMeO). Representatives of the firstsystem are, for example, silver-nickel (AgNi) or silver-iron (AgFe);representatives of the second system are, in particular, silver-cadmiumoxide (AgCdO) and silver-tin oxide (AgSnO₂). Further metal oxides suchas, in particular, bismuth oxide (Bi₂ O₃), copper oxide (CuO) and/ortantalum oxide (Ta₂ O₅) may also be added.

The practical suitability of a contact material based on silver-metal orsilver-metal oxide is determined by the so-called electrical "contactproperty spectrum". Important parameters in this context include thelifetime number of make-break operations on the one hand, which isdetermined by the erosion of the contact piece, and the so-calledtemperature rise on the other hand, i.e. the contact heating at thecontact bridge and at the terminals which essentially results from theelectrical resistance of the said contact construction. Also importantare a sufficiently low welding tendency of the contact pieces andadequate corrosion resistance. For it should be noted that, owing tolong-term corrosion of the material in air-break switching devices theswitching properties may change over time.

DE-A-1 608 211 discloses an electrical contact material of the systemsilver-metal oxide which, in addition to cadmium or tin oxide, may alsocontain iron oxide. Further, DE-C-38 16 895 discloses the use of asilver-iron material containing from 3 to 30% by weight of iron andcontaining one or more of the additives manganese, copper, zinc,antimony, bismuth oxide, molybdenum oxide, tungsten oxide, chromiumnitride in amounts of, in total, from 0.05 to 5% by weight, theremainder being silver, for electrical contacts. In addition, DE-A-39 11904 discloses a powder-metallurgical process for preparing asemi-finished product for electrical contacts from a silver-basedcomposite material containing iron, in which from 5 to 50% by weight ofiron as the first minor constituent and from 0 to 5% by weight of asecond minor constituent are used. The second constituent contains oneor more substances from the group comprising the metals titanium,zirconium, niobium, tantalum, molybdenum, manganese, copper and zinc aswell as their oxides and their carbides. The iron in elemental form isobtained in the process by chemical precipitation, in particular.Finally, JP-A-1/055345 discloses a material of the type mentioned at theoutset, which is composed of from 0.5 to 20% by weight of iron oxideparticles dispersed in a silver matrix, in which material a portion ofthe iron oxide is replaced by at least one of the oxides of nickel,cobalt, chromium, molybdenum, tungsten, cadmium, zinc, antimone, tin,bismuth, indium, lead, manganese, beryllium, calcium, magnesium orcopper. The contact pieces manufactured therefrom are claimed to bedistinguished, for use in switches, by good mechanical properties andhigh arc resistance.

WO-A-92/22080 discloses a contact material in which, in addition to theiron oxide, rhenium oxide and/or bismuth zirconate and/or boron oxideand/or zirconium oxide are present as a further effective component, theiron oxide being present as the main component in percentages by weightbetween 1 and 50%, and the minor components being present in percentagesby weight between 0.01 and 5%. The iron oxide may here have theconstitution Fe₂ O₃ or Fe₃ O₄ or alternatively, if required, be of mixedform.

In the materials of the prior art, all the requirements of the "contactproperty spectrum" are usually not met at the same time. The ultimateaim is to achieve, for each particular application, an appropriateoptimum of the parameters most important for that particular case.

SUMMARY OF THE INVENTION

Based on the prior art, the object of the invention is to find furthercontact materials based on silver-iron oxide and containing other minorcomponents, and to specify the appertaining preparation process. Thenovel materials are to be distinguished by low contact heating withstable heating behaviour, low tendency to welding and a long servicelife with respect to the breaking current intensities. Furthermore, goodcorrosion resistance should obtain.

The object is achieved according to the invention by the furthereffective component being an oxide of an element of the third sub groupof the Periodic Table of the Elements. In addition, at least one oxidemay be present which contains elements of the sixth subgroup of thePeriodic Table of the Elements. These contact materials are suitable, inparticular, for use in a low-voltage circuit breaker.

DETAILED DESCRIPTION OF THE INVENTION

Within the scope of the invention, the iron oxide is preferably presentin percentages by weight of from 3 to 20%, and the further effectivecomponent in percentages by weight of from 0.1 to 10%. In this context,the iron oxide may in particular be present either in percentages byweight of from 7.5 to 15% or in percentages by weight of from 4 to 7.5%,while the further oxide preferably has a percentage by weight between0.5 and 2%. The element for the effective component may be selected fromthe third subgroup of the Periodic Table of the Elements as one of theelements scandium (Sc), yttrium (Y) and lanthanum (La) with the furtherlanthanides. In particular, however, the further effective component isyttrium oxide (Y₂ O₃). Particularly good compliance with therequirements posed is provided by a material having the compositionAg/Fe₂ O₃ 10/Y₂ O₃ 1.

As a further metal oxide in a material of the constitution Ag/Fe₂ O₃ /Y₂O₃, ferrous tungstate (FeWO₄) may in particular be present. Particularutility is provided by a material of the composition Ag/Fe₂ O₃ 9/Y₂ O₃1/FeWO₄ 0.5.

The preparation of the novel contact materials is effected according tothe invention by silver powder and iron oxide powder first being mixedin the predetermined ratio, by this mixture being admixed with thefurther metal oxides, and by further processing then taking place byalternate sintering and pressing.

The invention provides improved AgNi substitute materials. The highercontact piece erosion in particular, observed in the known substitutematerials using iron oxide as the main effective component, in which theiron oxide fraction was chosen to be in particular below 7.5% by weightin order to ensure the temperature behaviour, is now reduced andapproaches the erosion of the silver-nickel.

It was found, within the scope of the present invention, that especiallyiron oxide as the main effective component in conjunction with yttriumoxide as the minor component improves the complete "contact propertyspectrum" of the contact material.

Further details and advantages of the invention can be deduced from thefollowing description of working examples. This includes a discussion ofthe accompanying Table containing individual examples for concretematerial compositions according to the invention.

In the Table, measured values for the temperature rise of the novelmaterials, which in each case was measured at the contact bridge of theswitching device, are given as maximum and mean bridge temperatures, aswell as measured values for the erosion behaviour.

The first column contains the maximum temperature rise in each case,arising in the bridge having the highest temperature values, the secondcolumn containing the mean values of all the temperature measurements.The values in each case result as the temperature difference withrespect to the room temperature. The third column lists values for theerosion which is calculated from weight measurements. All measurementswere carried out in switching sequence tests in a 15 kW contactor up toa number of make-break operations of n_(s) =50,000 switching operations.

The Table contains, in addition to reference materials, a typicalexample with a meaningful composition of the contact material accordingto the invention. Measurements were carried out in each case directly ona contact bridge of the 15 kW test contactor, using two contact piecesin each case. The measurement results are discussed below in moredetail.

EXAMPLE 1 Ag/Fe₂ O₃ 10/Y₂ O₃ 1 Contact Material

A material having the composition Ag/Fe₂ O₃ 10/Y₂ O₃ 1 is to beprepared. To this end, separate silver powder and iron oxide powder arefirst mixed in a predefined ratio, and this powder mix is admixed with1% by weight of yttrium oxide powder. The further preparation is carriedout in a known manner by alternate sintering and pressing underpredefined constraints.

The contact pieces are produced either by an extrusion technique or amoulding technique. In both cases, the rear sides, in order to ensurereliable connection, are provided, even as they are being fabricated,with a solderable and/or weldable silver layer. The contact pieces thusfabricated, having the constitution Ag/Fe₂ O₃ /Y₂ O₃, were subjected inthe contactor specified to comparison tests with known contactmaterials, whose results are depicted in the Table.

The Table first shows an AgNi20 contact material whose properties, bothwith respect to the bridge temperature and with respect to erosion areknown to be good. These values are made considerably worse in AgFe₂ O₃contact materials containing only iron oxide as a substitute for nickel,in particular the maximum temperatures observed at individual switchbridges being unacceptably high. At the same time, a rise proportionalto the oxide content is observed, whereas the erosion is reduced asexpected.

From the prior art, various further additives are already specified as aminor effective component which should improve these properties. Goodutility has been shown especially by zirconium oxide (ZrO₂), the resultsbeing listed in the Table. Here, with a low iron oxide content,especially at below approximately 7.5% by weight of Fe₂ O₃, a lowmaximum bridge temperature and an excellent mean bridge temperature areobserved, the erosion admittedly being unsatisfactory. The latter dropsonly at higher iron oxide contents, i.e. from approximately 7.5% byweight of Fe₂ O₃, a deterioration of the temperature behaviouradmittedly being observed at the same time.

It can further be seen from the Table that the temperature behaviour isimproved surprisingly, in particular by the addition of yttrium oxide,without the erosion becoming significantly worse. In this context, theyttrium oxide proportion may be in the range from 0.1 and 10% by weight.

The material Ag/Fe₂ O₃ 10/Y₂ O₃ 1 listed in the Table, in particular, isdirectly comparable in its temperature behaviour with the materialAgNi10. In contrast to the previously proposed AgFe₂ O₃ -based materialscontaining other minor components, the erosion is now, however,significantly below the erosion in the known materials. For example, fora number of make-break operations of 50,000 the erosion observed wasonly about 20% above that of silver-nickel, whereas it is considerablyhigher in the other substitute materials.

Owing to, in particular, the properties, equally suitable with respectto the temperature behaviour and the erosion behaviour, of Ag/Fe₂ O₃10/Y₂ O₃ 1, the conditions now exist for replacing the silver-nickelmaterials having the known adverse effects for a wide range ofapplications. Instead of, in particular, yttrium oxide the otherchemical elements of the third sub-group of the Periodic Table of theElements are also possible as the minor component of an AgFe₂ O₃material.

EXAMPLE 2 Ag/Fe₂ O₃ 9/Y₂ O₃ 1/FeWO₄ 0.5 Contact Material

In order to prepare this material, separate silver powder and iron oxidepowder are first mixed in a predefined ratio, and this powder mix isadmixed with 1% by weight of yttrium oxide powder and 0.5% by weight ofiron tungstate powder. The further preparation is carried out in a knownmanner by alternate sintering and pressing under predefined constraints.

Contact pieces are fabricated from this material either by an extrusiontechnique or a moulding technique. In both cases, the rear sides areprovided, during the fabrication of the contact pieces, with asolderable and/or weldable silver layer to ensure reliable connectionsof the contact pieces.

The contact pieces thus produced were subjected to comparative testswith, on the one hand, known contact materials and, on the other hand,the materials according to the other examples of the above description.

It was found that, using the material according to Example 2, it ispossible not only to attain the suitable temperature properties of thesintered contact materials, already proposed in Example 1, having theconstitution Ag/Fe₂ O₃ /Y₂ O₃, but also in particular to improve furtherthe welding and short-circuit behaviour. This material is thereforeespecially suitable for use in circuit breakers.

                  TABLE                                                           ______________________________________                                                 Maximum  Mean      Erosion of the                                             bridge   bridge    entire bridge                                              temperature                                                                            temperature                                                                             (two contact pieces)                                        K.!      K.!       weight loss in g!                                ______________________________________                                        AgNi20      91        68        0.13                                          AgFe.sub.2 O.sub.5 6.4                                                                   162        71        0.27                                          AgFe.sub.2 O.sub.3 10                                                                    168        87        0.18                                          AgFe.sub.2 O.sub.5 5.4ZrO.sub.2 1                                                         89        66        0.24                                          AgFe.sub.2 O.sub.5 10ZrO.sub.2 1                                                         136        78        0.16                                          AgFe.sub.2 O.sub.3 10Y.sub.2 O.sub.3 1                                                   101        74        0.16                                          ______________________________________                                    

What is claimed is:
 1. A composition comprising: silver, iron oxide, andat least one additional oxide of an element of the third subgroup of thePeriodic Table of the Elements.
 2. The composition according to claim 1wherein the iron oxide is present in an amount of about 3 to 20% byweight and the additional oxide is present in an amount of about 0.1 to10%.
 3. The composition according to claim 1 wherein the iron oxide (Fe₂O₃ /Fe₃ O₄) is present in an amount of about 7.5 to 15% by weight. 4.The composition according to claim 3 wherein the iron oxide (Fe₂ O₃ /Fe₃O₄) is present in an amount of about 9 to 12% by weight.
 5. Thecomposition according to claim 4 wherein the iron oxide (Fe₂ O₃ /Fe₃ O₄)is present in an amount of about 4 to 7.5% by weight.
 6. The compositionaccording to claim 1 wherein the additional oxide is yttrium oxide (Y₂O₃).
 7. The composition according to claim 6 wherein the yttrium oxideis present in an amount of about 0.5 to 5% by weight.
 8. The compositionaccording to claim 1 comprising Ag/Fe₂ O₃ 10/Y₂ O₃
 1. 9. The compositionaccording to claim 1 further comprising at least one further metal oxideof an element of the sixth subgroup of the Periodic Table of theElements.
 10. The composition according to claim 9 wherein the furthermetal oxide is ferrous tungstate (FeWO₄).
 11. The composition accordingto claim 10 wherein theferrous tungstate (FeWO₄) is present in an amountof about 0.1 to 1% by weight.
 12. The composition according to claim 9comprising AGFE₂ O₃ 9/Y₂ 1/FeWO₄ 0.5.
 13. Process for preparing acomposition according to claim 1 comprising the steps of:(a) mixingpowders of silver and iron oxide in a predetermined ratio; (b) admixingat least one oxide of an element of the third subgroup of the PeriodicTable of the Elements; and (c) alternately sintering and pressing themixture.
 14. The process of claims 13 further comprising the step ofadmixing at least one further metal oxide of the an element of the sixthsubgroup of the Periodic Table of the Elements prior to step (c).
 15. Amethod of using the composition according to claim 1 comprisingfabricating the composition into a contact piece for a switching device.16. The method of claim 15 wherein the switching device is a low voltageswitch.
 17. The method of claim 15 wherein the composition is extruded.18. The method of claim 15 wherein the composition is moulded.